1. Field of the Invention
The present invention relates to a wireless communication terminal apparatus having a non-contact IC card function (RFID (Radio Frequency IDentification) function of an electromagnetic induction method) and to a method of adjusting the resonance frequency of an antenna for a non-contact IC card installed in the wireless communication terminal apparatus.
2. Description of the Related Art
In recent years, non-contact IC cards that can be used for various kinds of applications, such as for transportation commuter passes, logging of employees arriving/leaving the office, electronic money, and credit cards, have become popular. Furthermore, functions of such non-contact IC cards have now been incorporated in some mobile phones, and such mobile phones can be used to settle electronic money transactions.
In such a non-contact IC card, it is common practice that, when carrier waves are to be received from a reader-writer (hereinafter referred to as “R/W”), reception efficiency is improved by performing parallel resonance using self-inductance values (hereinafter abbreviated as “L values”) possessed by a loop antenna and capacitors connected in parallel to the loop antenna. The parallel resonance frequency (hereinafter F0) is an optimum value in the vicinity of a carrier frequency (13.56 MHz), however, variations in F0 occur due to various factors.
First, variations in F0 may be caused by variations in the capacitances of capacitors that are connected in parallel. That is, since the relationship between the parallel resonance frequency and the capacitor capacitance is determined by F0=½π√(LC), variations in capacitor capacitance are directly related to variations in F0.
Furthermore, variations in F0 may be caused by the positional relationship between an antenna and a peripheral metal. That is, in a mobile terminal in which a non-contact IC card is incorporated, in general, a metal is often used, such as for fill-patterns of a circuit substrate, a battery pack, a magnesium alloy for ensuring the strength of the housing, and a SUS (Steel Use Stainless) plate for a shield. This metal exists in an AC magnetic-field, and on the surface of the metal, eddy current occurs in a direction that cancels a change in the magnetic field. Then, magnetic-flux changes forming the self-inductance of the antenna are cancelled by the eddy current, and the L value of the antenna is decreased when a metal substance approaches the antenna. Therefore, variations in the positional relationship between the metal substance and the antenna are directly related to variations in F0.
Furthermore, variations in F0 may be caused by variations in the magnetic permeability of a magnetic substance (when a magnetic substance is used in the vicinity of the antenna). That is, canceling of changes in the magnetic flux due to eddy current causes carrier waves from the R/W to be attenuated and causes communication performance to be deteriorated. In order to reduce the attenuation and performance deterioration, in a mobile phone in which a non-contact IC card function is incorporated, a magnetic substance having a high magnetic permeability is often provided between an antenna and a metal. As a result, the magnetic flux that impinges on a metal is decreased, and the influence of the eddy current is reduced. On the other hand, the magnetic-flux density in the vicinity of the antenna is increased in proportion to the magnetic permeability. Then, when the magnetic-flux density forming the self-inductance is increased, the L value is increased. Therefore, variations in the magnetic permeability lead to variations in F0.
Also, variations in F0 occur due to the positional relationship between the antenna and the magnetic substance and due to the dimensional accuracy of the antenna. That is, the former case is related to the fact that, when the magnetic substance approaches the antenna, the L value is increased, and when the magnetic substance moves away from the antenna, the L value is decreased. In the latter case, variations in the L value due to variations in the pattern length and pattern intervals of a loop antenna are factors affecting F0.
When F0 becomes a value different from the carrier frequency due to the above-described variation factors, the communication distance becomes considerably decreased. Furthermore, since the difference between F0 and the carrier frequency appears as a phase difference between a transmission wave and a received wave of an R/W, an ASK modulation width disappears in the antenna of the R/W when a particular phase difference is reached, and a dead zone called a null occurs. In this state, since the card function side receives a carrier wave of a sufficient strength, power-on reset does not occur, and there is a case in which it is not possible to recover from an error state until a user removes the non-contact IC card so as to be outside the carrier frequency range.
In a mobile phone in which, in particular, a large amount of metal is used, since the amount of influence exerted by the metal of the mobile device on the R/W side needs to be taken into consideration, it is necessary to manage the F0 of the mobile phone at as narrow a band as possible. For this reason, adjustments for allowing the narrowing of F0 within a narrow band become necessary. In particular, in order to eliminate variation factors due to the positional relationship among the above-described variation factors, a mechanism capable of adjusting the resonance frequency after the housing of the mobile terminal is assembled in manufacturing steps becomes necessary.
Examples of a method of adjusting a resonance frequency in the related art include a method using a cut pattern. This is a method in which, for example, as shown in FIG. 1, capacitors of 1 pF, 2 pF, 4 pF . . . 2n pF are arranged in parallel to each other, and a capacitance is selected in steps of 1 pF in the range of 0 pF to (2n−1) pF by cutting a pattern between capacitors and an antenna. However, in this method, a large number of operation steps or large facility costs are incurred. Furthermore, since it is difficult to restore the pattern to its original form after being cut, there is a drawback in that it is difficult to reuse a substrate when it fails in a manufacturing inspection of another item and maintenance is performed. Furthermore, since a hole through which cutting is performed after the non-contact IC card is set in the housing of the mobile terminal becomes necessary, there is a drawback in that this imposes limitations on mechanism design and mobile terminal design. In addition, another drawback is that it is difficult to place parts around the cut place, and in the case of a multilayered substrate, the internal layer pattern at the cut place may be damaged.
Other examples of a method of adjusting a resonance frequency are mechanical adjustment methods. As shown in FIG. 2, such mechanical adjustment methods include a method of switching on or off parallel capacitors by using a dip-SW in place of a cut pattern and a method of performing F0 adjustments by turning a trimmer capacitor. These methods have a drawback in that a large number of operation steps are taken, and when the methods are performed after the housing is assembled, a hole for adjustment operation is necessary, thereby imposing limitations on mechanism design and mobile terminal design. Another drawback is that the dip-SW and the trimmer capacitor are typically of a size which allows operation by a person and which needs a large mounting area.
Accordingly, a method using a switching element has been considered. This is a method of using a switching element, such as an FET, instead of a dip-SW, as shown in FIG. 3 (refer to “University Lectures Up-To-Date Electrical Machinery & Apparatus”, Revised and Enlarged Edition, October 1996, Author: Shota MIYAIRI, Publisher: Maruzen).